That is the long term plan, as far as finances allow. The short term was to get hold of the widest lens available in the prime lenses, something that even with the 1.6X magnification factor would give awesome wide angle.

The lowest I've found is 12mm lenses (19.2mm with crop factor), even of these i have no idea which ones are good

Are you sure you have a Sony, not a Canon? Because Sony's APS-C crop factor is 1.5, Canon is 1.6.

Anyway, adapting legacy wides is not such a good idea as modern lenses are just so much better. There's a very nice modern Samyang 12mm and it's affordable. It's sold under various different names around the world, you should look into that.

If you're also not rectilinear wideangles, there are also a few fisheyes available. As far as I know you get the widest angle of view with the Meike 6.5mm which is a circular fisheye that can actually see slightly behind itself (190° diagonal).

Like Miran, I would advise against adapting legacy wide angle lenses. Ultra wide lenses were difficult and expensive to manufacture until recent years. So the legacy ultra wide angle lenses are also rare, expensive and not very good.

You seem to have been looking at the newer manual lenses with native E mount, like Samyang. That's probably your best bet. If you want better quality than the kitlens but not necessarily a wider field of view, then there are a few options like Sony 20 or Sigma 19. You could also look for a used copy of the Sony 10-18.

Speaking of rectalinear glass I got Laowa 12mm f2.8, with focal reducer it's 8.5mm and works fine, just lean a bit forward to avoid your shoes in the picture. There's also Irix 11mm (7.8mm with focal reducer) which should be good thou I have not tried one. Viltrox makes a good focal reducer at fraction of the price of Metabones..